Fixed warnings

``` In file included from vi.c:16: ./vi.h:343:18: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes] 343 | void led_modeswap(); | ^ | void In file included from vi.c:20: ./led.c:396:18: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes] 396 | void led_modeswap() | ^ | void 2 warnings generated.